This invention relates to a method of forming a coil spring including feeding an element wire along its longitudinal direction for forming a coil spring.
Generally known are several methods of forming a coil spring with a desired pitch and diameter by running an element wire in the aforesaid manner. As an example, there is a method in which the element wire is spirally wound around a core member. This method, however, is subject to the following drawbacks. First, in hot-forming e.g. a barrel-shaped spring, the formed spring cannot be separated from the core bar unless the core member is axially displaced for approximately half a pitch and rotated, so that the manufacture and handling of the core member requires special care. Secondly, the formation of end turn portions with a pitch different from that of the principal part of the coil spring requires a separate process. In order to remove such awkwardness, there is proposed the use of a bevel core member whose external shape can be changed by means of a suitable link mechanism. With use of such core member, however, produced coil springs are liable to become polygonal. Further known is a method of manufacturing a coil spring without using any core member. In this method, an element wire running along its longitudinal direction to be curved is pressed against coiling points arranged substantially at right angles to the running direction. Although having many advantages, this method is also subject to some drawbacks as follows. First, this method requires a strongly-built coiling machine because of great force being applied to the coiling points when using an element wire with a large diameter (e.g. 10 mm or more). Secondly, manufacture of only one coil spring requires an element wire with a length corresponding to at least several springs to be fed into the coiling machine, so that the formed coil spring, as well as the coiling machine, cannot help being costly. Thirdly, with scratches or gashes no formed coil springs produced by a hot working process are adaptable for practical use.